In recent years, due to environmental issues and resource issues, low heat buildup has been strongly demanded in the tires of automobiles. Further, from the viewpoint of durability, excellent abrasion resistance has been sought. A tire which is obtained from a rubber composition which contains silica is superior in low heat buildup compared with a tire which is obtained from a rubber composition which contains the usually used carbon black, so this can be used to produce a tire helping to realize lower fuel consumption.
In such a rubber composition, to raise the affinity of the rubber and silica, the art is known of making a modifier react with the polymerizing active ends of the rubber etc. so as to introduce functional groups with a high affinity to silica.
For example, Patent Document 1 discloses using an organolithium catalyst which is prepared from a polyvinyl aromatic compound and lithium in a predetermined molar ratio so as to polymerize a conjugated diene-based monomer, reacting a modifier with the obtained polymerizing active ends, and adding silica or other filler to the thus modified conjugated diene-based rubber to obtain a rubber composition. However, in the art of the above Patent Document 1, while the affinity of the rubber and silica is improved, there are the problems that the low heat buildup property is not sufficient and, further, the abrasion resistance is inferior. Therefore, when used as rubber for a tire, the required properties can not necessarily be satisfied.
As opposed to this, for example, Patent Document 2 discloses a method of production of a radial conjugated diene polymer which uses an alkali metalated aromatic compound which hays three or more carbon atoms which are directly bonded to alkali metal atoms and aromatic rings in one molecule as a polymerization initiator and polymerizes a monomer mixture which contains at least a conjugated diene compound. In this Patent Document 2, the conjugated diene polymer is made one which has a radial structure so as to improve the affinity with silica or other filler when adding the filler and thereby enable improvement of the low heat buildup and abrasion resistance.